1. Field of the Invention
The present invention generally relates to oil and gas well (borehole) logging tools, specifically to an improved method of measuring resistivity of geologic formations, and more particularly to a method of logging-while-drilling using a dual-transmitter, dual-receiver, or multi-pair receiver, resistivity measuring collar.
2. Description of Related Art
Logging tools for measuring earth formation properties are well known, particularly those used in the location of underground petroleum products (oil and gas). Borehole logging instruments use various techniques to determine geophysical properties such as bulk density, resistivity, porosity, etc. From these properties, the lithology of the surrounding formation can be predicted, i.e., whether the predominant minerals are sandstone, limestone, dolomite, etc., which provides an indication of the likelihood of the presence of petroleum products or hydrocarbons in the formation.
Techniques for ascertaining formation properties include those involving the use of radiant (electromagnetic) energy. For example, gamma rays are commonly used to measure bulk density of a formation by detecting such radiation as it passes through the formation, and relating the amount of detected radiation to the electron density of the formation. Lower energy techniques, such as those using frequencies around 2 MHz, can be used for resistivity logging via electromagnetic induction or electromagnetic wave propagation. Porous formations having high resistivity generally indicate the presence of hydrocarbons, while those having low resistivity are often water saturated.
Logging while drilling (LWD) is often run in lieu of wireline logging tools. This approach is more cost effective than the somewhat repetitive steps of drilling a well, and afterwards logging the well with an induction wireline tool. More importantly, logging while drilling allows the drilling team to better direct the drill string, that is, dynamically adjust the depth, attitude, inclination, etc., based on concurrent data analysis. LWD resistivity tools operating at relatively higher frequencies (e.g., 2 MHz) are easier to implement than low frequencies, but at higher frequencies the skin effect is more pronounced than at lower frequencies, making the measurement more sensitive to the resistivity of the formation only a few inches away from the borehole. Wireline induction tools which operate generally or traditionally at lower frequency (e.g., 20 KHz) have a greater range and better accuracy.
It has been known for a long time that for an electromagnetic wave propagating in a conductive medium, such as a geological formation, the higher the conductivity, the higher the phase shift measured at two receivers placed away from the transmitter, (see U.S. Pat. Nos. 3,551,797 and 3,849,721). Similarly the higher the conductivity, ie., the lower the resistivity, the larger the attenuation measured between two such receivers. The main reason people measure both phase and attenuation is because it has been shown, and generally accepted in the industry that the attenuation measurement sees deeper in the formation, away from the borehole, than the phase measurement. See U.S. Pat. No. 4,185,238. The combination of both phase and attenuation provides a visual indication of the existence of invasion, since both measurements will coincide in non-invaded formations and will separate in invaded formations. The dielectric constant also affects the phase and attenuation and plays a parasitic role in the whole process.
One commonly used design provides two outer transmitters and two inner receivers, as shown in FIG. 1, and described in U.S. Pat. No. 4,899,112. The upper and lower transmitters TU and TL are sequentially energized, and the phase difference and attenuation between the receivers RU and RL are recorded for each transmitter. The phase differences and attenuations are averaged with the proper polarity, and these average values used to determine resistivity.
One problem with this approach is that the distance between transmitters should be relatively large, meaning that power must be transported a relatively large distance to the transmitters (if a single power source is to be used), and the attendant ohmic losses increase power consumption. Downhole power is at a premium in LWD, and many LWD subassemblies are energized by expensive lithium batteries (it would be more problematic to provide separate power sources for the transmitters).
Other commercially available resistivity tools use a dual or multiple spacing coil array similar to that shown in the xe2x80x2112 patent. The reason for multiple transmitter pairs is to investigate at a plurality of radial distances away from the borehole axis. Typically the further the transmitter from the receiver, the deeper the reading, until such time where the skin effect limits the depth of investigation. Also for a given transmitter receiver spacing, it has been shown that the depth of investigation is generally larger for attenuation derived resistivity measurement than for phase derived resistivity measurement. As seen in FIG. 2, four outer transmitters are used with two inner receivers. With this construction, all four transmitters (upper and lower shallow transmitters TUS and TLS, and upper and lower deep transmitters TUD and TLD) are again sequentially energized. For each of the four states in the energization cycle, the phase differences and attenuations between the upper and lower receivers are recorded, and these phase differences and attenuations are again averaged with the proper polarity.
A multiple spacing coil array system such as that shown in FIG. 2 not only suffers from the same problems as the configuration of FIG. 1, but further takes longer to perform a full cycle, and requires more electrical energy to power the four different states. For multiple spacings beyond two, the time and energy penalties increase even more.
In light of the foregoing, it would be desirable to provide an improved method and downhole tool for resistivity measurements. It would be further advantageous if the method and tool were easily adapted for logging while drilling.
It is therefore one object of the present invention to provide an improved method and device for resistivity logging of geologic formations.
It is another object of the present invention to provide such a method and device that have reduced power requirements, while still maintaining accurate resistivity measurements.
It is yet another object of the present invention to provide such a method and tool that can advantageously be applied to logging while drilling (LWD).
The foregoing objects are achieved in an LWD resistivity device generally comprising at least two inner transmitters surrounded by at least two outer receivers, wherein each transmitter is sequentially excited, and measurements taken at each of the two receivers. The voltages at the two receivers will be sinusoidal signals with the same frequency as the given transmitter and the phase difference between these two signals is determined, once for each transmitter. The two values will then be averaged. The average phase difference between the two receivers is used to estimate the resistivity value of the surrounding formations. This construction requires less power to operate than prior art systems. Similarly, the attenuation of the signal between these two receivers is determined once for each transmitter. The two values corresponding to the two transmitters respectively are then averaged. The average attenuation between the two receivers is used to estimate the resistivity value of the surrounding or adjacent earth formation. Mathematical modeling shows that the depth of investigation of the attenuation derived resistivity measurement is generally greater than the depth of investigation of the phase derived resistivity measurement.
In a multiple coil spacing embodiment, four outer receivers are used with at least two inner transmitters, there being two receivers on each end of the collar. Measurements for multiple receivers can be carried out simultaneously, allowing faster logging, and providing further energy savings. Symmetrical or nearly symmetrical receivers"" data are averaged for the two transmitters. Mathematical modeling shows that the longer the spacing between the transmitter and the receiver, the deeper is the investigation of the device, both for the phase and the attenuation derived resistivities. Therefore, multiple pairs of outer receivers yield multiple depths of investigation for the phase derived resistivity and multiple depths of investigation for the attenuation derived resistivity.
In a further embodiment, an azimuthal receiver array is used to identify nearby or approaching boundary transitions.
The above as well as additional objectives, features, and advantages of the present invention will become apparent in the following detailed written description.